1. Field of the Invention
The present invention relates to a process for the preparation of silica glass powders and, more particularly, to a process for the preparation of silica glass powders with a simple operation and with high efficiency, which requires no mechanical pulverization step so as avoid contamination of impurities and which can provide highlY purified silica glass powders capable of being appropriatelY used for various purposes, such as raw materials of silica glass products for the semiconductor industry and multicomponent glass for optical communication.
2. Description of Related Art
Recently, there have been increasing demands to purify silica glass powders to be used, for example, as raw materials for silica glass products for the semiconductor industry and multicomponent glass for optical communication.
At present, demands have been made to provide a silicon single crystal of an extremely high purity as integration of semiconductor elements develops to a greater extent, so that highly purified crucibles for preparing such highly purified silicon single crystals have also been demanded.
Heretofore, the crucibles used for the preparation of silicon single crystals for semiconductors have been prepared by melting finely pulverized powders of natural quartz and sintering them in a carbon arc furnace. The finely pulverized powders of natural quartz, however, suffer from contamination of impurities such as aluminium resulting from alumina ball mills used in the pulverizing step, on tope of impurities contained in the natural quartz itself. The crucibles may usually contain impurities such as Al as high as 10 ppm, Fe, Ti, Zr, Na, Ca and K as high as several ppm, as well as B, Cu and Ni as several 1Oth ppm. If these crucibles are used to prepare silicon single crystals, the impurities contained therein may be transferred to the silicon single crystal, thereby reducing the purity of the silicon single crystal. As a result, a silicon single crystal of an extremely high purity can not be provided.
Further, a transparent silica glass tube of high purity has been demanded for a furnace core tube etc. in the thermal processing step of producing semiconductor wafers. The transparent silica glass tubes have conventionally been prepared from a natural rock crystal containing minute impurities such as aluminium, alkali, etc., so that the transparent silica glass tubes resulting therefrom may also contain such impurities. Hence, conventional silica glass tubes present the problems of heat transformation and lack of transparency resulting from contamination with those impurities.
It is generally known to the art that heat transformation and lack of transparency in silica glass material may occur to a remarkable extent in instances where an alkali metal is present as an impurity. In this respect, a high purity of silica glass powder as a raw material is required.
Mechanical pulverization of natural quartz or synthetic quartz for the preparation of powders of the raw material presents the problem that it can provide the powders in a range of particle sizes from 75.mu.m to 300.mu.m in a yield as low as 50% or lower, while the rest is fine powders having particle sizes of 75 .mu.m or smaller.